Star-branched polymers, also known as radial polymers, are characterized by having three or more polymeric arms emanating from a central core. These polymers can be prepared by various polymerization procedures such as anionic, cationic, and free radical mechanisms. The star polymers are usually formed by using either multifunctional initiators, multifunctional chain transfer agents, or multifunctional coupling agents. The star polymers have unique properties including: narrow molecular weight distributions; low viscosities at low molecular weights or in solution due to their compact structures; and high viscosities at high molecular weights due to extensive entanglements.
U.S. Pat. No. 3,769,254 discloses pressure sensitive adhesives prepared from linear polymers of acrylic polymers containing monomers with reactive hydrogen groups. In general, high molecular weight linear polymers are usually used in high performance applications. Such polymers have solution viscosities proportional to their molecular weight. Attempts to lower the solution viscosities by reducing molecular weight results in a reduction in adhesive properties which makes them difficult to use as hot melts for high performance and coating applications. In linear systems, these lost properties cannot be recovered by crosslinking.
U.S. Pat. No. 5,679,762 discloses block star polymers which overcome some of the problems associated with a linear system. This patent is limited to block polymers which provide phase separation to attain the desired adhesive properties.